Water/diesel oil emulsions are nowadays known and used in diesel cycle internal combustion engines and in heating burners with the purpose of reducing the presence of harmful pollutants in exhaust gases, such as NOx, SOx, CO and particulate matter (PM).
The practice of adding water to diesel dates back to the early 1900s, and there are hundreds of different patented implementations. Depending on the application, the water content may vary from 8%-35%, with a typical 13% water formulation for diesel engine use. There are numerous benefits to adding water to diesel. Water vaporization increases fuel dispersion in the form of smaller droplets and the contact surface between fuel and air is increased. As a result, combustion is more efficient; combustion temperature peaks are lowered, thereby reducing NOx; and PM formation is reduced.
Document WO 2013/124726 shows an installation and a process for preparing a water/diesel oil emulsion. The installation comprises: a water feeding unit, an activator feeding unit, a diesel oil feeding unit and a pre-mixing tank. A mixing device is placed downstream of said pre-mixing tank and comprises: a tubular duct defining an extension direction, at least one helical channel arranged inside the tubular duct for generating a turbulent flow of the mixture of water, activator and diesel oil inside said tubular element, at least one first baffle comprising a plurality of holes and arranged perpendicularly to said extension direction and downstream of said first helical channel, at least one inlet element comprising a tapered portion at an end thereof and a plurality of holes arranged with an axis of extension substantially parallel to the axis and adapted to let the inlet fluid into the first helical channel. Document EP 0 958 853 discloses a process for producing stable emulsions of at least two substantially immiscible fluids, particularly emulsions of a liquid fuel with water. The fluids to be emulsified are injected into an emulsification chamber provided with an injection system which imparts to the fluids a motion in a direction which is substantially perpendicular to the general direction in which the fluids travel through the emulsification chamber. This injection system is provided by means of a diffuser having an inlet hole, through which a stream of liquid is fed in a substantially axial direction, and at least one outlet hole, which leads into the chamber and whose axis lies on a plane which is substantially perpendicular to the direction of the inlet stream. In this manner, the stream strikes the walls of the emulsification chamber, producing a turbulent fluid motion which has a predominantly helical orientation and is capable of producing a dispersion of one fluid in the other, forming dispersed-phase particles.
In this field, the Applicant has observed that the manufacturing process for emulsion fuels is not straightforward. Indeed, the emulsion fuel cannot be produced simply by putting together water and fossil fuel.
The Applicant further observed that the dispersion of fine water and oil droplets, which are inherently mutually immiscible, is very unstable and goes back to the initial condition of the two-phase mixture over time. This phase separation phenomenon makes it difficult to accurately evaluate combustion efficiency for individual emulsion fuels.
The Applicant further observed that, since the phase separation phenomenon gradually proceeds over time, the combustion efficiency of the particular fuels involved varies depending on the time when the combustion experiment is performed.
The Applicant further observed that, currently, ultrafine emulsion fuel products are available, although there has not been any technology to maintain the products in a stable emulsified state. Emulsifying agents may be added to protect oil and water droplets in order to maintain the emulsified suspension. The development of these agents, however, is still in the fledgling stage and presents several challenges, including the possible effects on combustion.
The Applicant finally observed that the most urgent problem is how phase separation can be avoided to produce optimal emulsions and how the emulsified condition can be stabilized during a longer period.
The Applicant further observed that the process and plants of the prior art, like WO 2013/124726 and EP 0 958 853 cited above, cannot guarantee the stability needed.
The Applicant perceived that an higher level of emulsification can be promoted by recirculating a batch comprising diesel oil, water and an emulsifying composition through a mixing device.
The Applicant has finally found that a mixing device provided with the features claimed and described further on is able to exert on the flowing liquid compression, centrifuging, dispersion and shear forces in order to promote emulsification.
The Applicant verified that this recirculation system allows to increase the emulsification to highest levels.
The Applicant further verified that this recirculation system allows to flexibly select the emulsification level of the batch, in particular adjusting the number of recirculation loops